Big end journal radius

rob...

the correct radius is important 090 is correct as you stated.

especially on a commando as the cranks whip.

using a drill as a gauge is not good enough.

and some machine shops which grind cranks for cars don,t always

dress there tool to the correct radius.

get your crank checked at one of the norton suppliers.

i use mike hemmings... good luck

tony

A too small radius will cause the crank to crack, too large would be ok provided the width of the journal will take the shell bearing but it does need some endfloat to take into account the bores being not exactly the right distance apart so careful measuring is needed. I have been having parts machined for 30 years, whatever the cleaning facilties are I always clean again myself as just even sitting on the shelf waiting for pickup will make a piece dirty again.

Previously Rob Bradley wrote:

Why is the radius so important? What are the implications if the radius is bigger than the .090" specified, as it is on my big ends, as well as in my case not being a smooth curve, except on the drive side outer?

Two highly recommended engineering shops have done shoddy work on this crank now, so how do I know who to trust? (I'm sure Mick Hemmings knows these engines inside out and does good work most of the time, but I met someone with a Commando who claimed to have had issues with grit in a crankshaft that he'd worked on, just as I did with SEP).

Previously bill_moline wrote:

Previously Rob Bradley wrote:

Why is the radius so important? What are the implications if the radius is bigger than the .090" specified, as it is on my big ends, as well as in my case not being a smooth curve, except on the drive side outer?

Two highly recommended engineering shops have done shoddy work on this crank now, so how do I know who to trust? (I'm sure Mick Hemmings knows these engines inside out and does good work most of the time, but I met someone with a Commando who claimed to have had issues with grit in a crankshaft that he'd worked on, just as I did with SEP).

Don't bag the crank grinder. He uses an emery wheel to grind your crank. It is the responsibility of the assembler to make sure parts fitted are clean and on size.

Any decent crank grinder will be aware of the need for the radius of the journal cranks web interface. This is a potential stress concentrator and can lead to crack propagation and bang!

Mercedes Benz made this fundamental error on their 2.5 desmodromic F1 engines of the 50's. The result was a series of crank breakages in the center of the 8 cylinder crankshaft. Not realizing the cause they decided to use 2* 4cylinders coupled in the center by a large gearwheel from which the power was taken off.

Its a Zombie thread from 2014.

Previously Rob Bradley wrote:

How important is this? One of my workshop manuals says "IMPORTANT: re-ground big ends should be radiused to 0.090"-0.094"

The attached images are of my recently-ground 88 crank. Are the radii acceptable? (The 3/32" drill is my version of a radius gauge). The second image is of the Drive side inner radius. Image management controls won't let me edit its title for some reason.

Hi Rob,

looking at it your attachments it would seem that your crank is close to becoming an expensive door stop.

It is critical when a crank is reground that the radius behind the bearing should be accurate, well finished and should blend in with both the reground journal and the crank cheek, if not, stress raisers will be produced and the crank will almost certainly break.

In your first, second and fourth attachment the radii are anything but that, in the third attachment the radius looks right but appears to step down to the journal rather than blend into it, this is not acceptable.

If you haven't already done so you need to return the crank to the grinder and ask for it to be corrected,( it is also important that the finish on the radius is as good as that on the journal)but they might be reluctant toreshape one of their wheels.

In the distant past I had a crank reground only to find on collection that the centre of the journal had been ground but not blended into the cheek, it took a great deal of persuasion and withholding payment before satisfaction was achieved.

I really wouldn't use this crank unless you can get this work corrected, if you can't then the only alternative is another crank.

Hope you get orby nowhave got some success with your crank.

Regards, Ian